208 HISTORICAL GEOLOGY 



water, such as flood-plains or lakes, where changing condition 

 frequently allowed the surface laj^ers to lie exposed to the 

 sun. 



"The curiously shaped and often huge Reptiles of that age 

 (Triassic) wandered over the mud exposed at low tide, and their 

 foot-prints, being covered by the deposit of the next flood tide, 

 constitute the so-called 'Bird tracks' which have been found in 

 such great numbers and perfection." x 



During the time of the formation of the Newark beds, there was 

 considerable igneous activity, as shown by the occurrence of sheets 

 of igneous rocks within the mass of sediments. In some cases true 

 lava flows with cindery tops were poured out on the surface and 

 then became buried under later sediments, while in other cases the 

 sheets of molten rock were forced up either between the strata or 

 obliquely through them, thus proving their intrusive character. 

 As a result of subsequent erosion, these igneous rock masses often 

 stand out conspicuously as topographic features. Perhaps the 

 most noteworthy of these is the great igneous rock sheet, part of 

 which outcrops to form the Palisades of the Hudson River, and 

 which altogether outcrops for a distance of 70 miles. The molten 

 rock first broke through the strata and then crowded its way along 

 parallel to them. Another fine example is the so-called Holyoke 

 Range of Massachusetts (Fig. 124) regarding which Emerson says: 

 "The accumulation of sediments was interrupted by an eruption 

 of lava through a fissure in the earth's crust, which opened along 

 the bottom of the basin. The lava flowed east and west on the 

 bottom of the bay, as tar oozes and spreads from a crack, and solidi- 

 fied in a sheet which may have been 2 or 3 miles wide and about 

 400 feet thick in its central part. This is the main sheet or Holyoke 

 diabase. The sheet was soon covered with sand layers, but its 

 thickness was such that it had shallowed the waters to near tide 

 level, and thus occasioned extensive mud flats." 2 In both regions 

 just mentioned, the contraction of the cooling masses often ex- 

 pressed itself by breaking the rock into great and small, crude, 

 nearly vertical columns, and hence the application of the term 

 "palisades." The steep mountain sides or cliffs are due to the fact 

 that the hard igneous rock is much more resistant to weathering 

 and erosion than the sandstone above and below it (Fig. 124). 



1 B. K. Emerson: U. S. G. S., Holyoke folio Xo. SO, p. 3. 



2 Ibid., Folio No. 59, p. 3. 



