THE ADIRONDACK MOUNTAINS 43 



The very latest Prepaleozoic rock in the whole Adirondack 

 region is another sort of dike rock known as diabase. It is really 

 almost the same as" basaltic lava, being fine grained, bluish black 

 where freshly broken and containing chiefly plagioclase feldspar, 

 pyroxene and iron ore (magnetite). It differs from the gabbro in 

 being finer grained and always occurring in sharply defined, narrow, 

 dike or fissure form. These dikes commonly vary from a few 

 inches or feet wide and a few rods long to many feet wide and 

 several miles long. Such dikes are of rather common occurrence 

 throughout the Adirondacks and are very easily recognizable by 

 their black, sharply defined outlines. The fact that they are co 

 fine grained, often even glassy at the borders, shows that they 

 cooled much more quickly and nearer the surface of the earth 

 than any of the other igneous rocks. It seems quite clear, there- 

 fore, that the diabase now at the surface was forced into fissures 

 after most of the Prepaleozoic erosion of the region had been 

 accomplished. On the other hand, the intrusions must have taken 

 place before the oldest Paleozoic strata around the Adirondacks 

 were formed because such strata have been observed to rest upon 

 the diabase. In the light of these facts it is safe to say that these 

 latest dike rocks are miUions of years younger than the oldest 

 igneous rocks of the Adirondacks. 



Paleozoic History 

 Cambrian period. After the first known Adirondack uplift, the 

 whole region was subjected to profound erosion which began long 

 before the opening of the Paleozoic era. The first period of the 

 Paleozoic era is known as the Cambrian. Since later Cambrian 

 strata rest upon the Prepaleozoic rocks around most of the borders 

 of the Adirondacks, and the earlier Cambrian strata are absent, 

 with no reason to think that they ever were deposited there, we 

 conclude that the time of profound erosion extended into the early 

 Paleozoic or, more strictly, into the later Cambrian. The reader 

 may inquire, Where are the sediments which were derived from the 

 wearing down of the ancient Adirondacks during this vast length 

 of time? Regarding the deposition of the Prepaleozoic sediments 

 we have no certain knowledge. They may have washed westward 

 or southwestward into waters which may possibly have existed 

 there ; they may have been carried northward or northwestward 

 into Canada to build up later Prepaleozoic strata there; or they 

 may have been- transported eastward toward or into the Atlantic 



