( 38 ) the earth is a very complex object,made up of many layers. (   39   )  is  only the  upper  surface  of  the  'skin',or  crust.This  crust  is  altogether  rather  more  than   100  km  deep. The  outer  crust,of a  depth  of  approximately  8  km,is  made  mostly  of very  hard  rock,a  kind  of granite.

( 41   ) He  says  riding  a bicycle to work  does not burn  fossil  fuel  or  create  dangerous pollutants. (   42   ) The   Department   of  Transportation  says  fifty  percent   of  American   drive eight kilometers  or  fewer  to  work.Shorter  car  trips  release  more  pollution  into  the  air  for  each kilometer  driven.This  is  because  the  device  in   a  car  engine  that  reduces  the  harmfulness  of emissions needs to warm up before it can work well.

( 43   ) For  example,Minnesota  Representative  James  Oberstar  is  a  stronger  supporter of  bicycle   use.He   says    cities,counties,state   governments    and   state    highway   transportation agencies are planning the roadways of the future.They are creating roads and paths for bicycles in cities and between communities.

Last  year,the  Pacific  Northwest  city  of  Portland,Oregon,had   the   highest  percentage   of bicycle  commuters  in  the  United  States.Portland  has  been  doing  progressive  city  planning  for many years to create special paths for many years to create special paths for bike riders.

Andrew  Land  is  one  of Portland's  citizens  who  bikes  to  work  every  day.Minister  Land  is thirty-three  years  old  and  has  never  owned  a  car. (   44    ) Before  moving  to   Portland  six years ago,he lived in Washington,D.C.But he was hit by a car twice while biking to work there. That has not happened in Portland where there are special roads for bicycles.