GEOLOGY OF THE NORTHERN ADIRONDACK REGION 345 



tively flat, tlioiigh no sucli striking instance bas' been met with as 

 Adams has described in Canada..^ The general foliation strike 

 in the Adinondacks lis n.e. and s.w., and the usual dip is to the 

 east, though there are many exceptions to both rules. In some 

 districts there is evidence of considerable folding of the bedding 

 and foliation planes but it is seldom sharp, and intricate fold- 

 ing and plioatiion occur seldom, if ever. Van Hise has sug- 

 gested that the development of foliation parallel to bedding 

 may have been initiated hj vertical shortening and horizontal 

 elongation below the level of no strain, or of no lateral stress, 

 and that in subsequent compression and folding the varying 

 strength of the different beds controlled the movement and 

 kept it in the same planes.^ This is an ingenious and very 

 plausible suggestion, the likelihood of which is emphasized by 

 the many evidences of the deep seated character of the meta- 

 morphism. But the uniformity of direction Oif foliation in both 

 sedimentary and igneous rocks shows that the metamorphism 

 which produced it followed the appearance of the igneous rocks, 

 and that it must have been produced in both at the same time. 



Late Precambric igneous rocks. Bikes of two sharply con- 

 trasted sorts of rocks are of frequent occurrence in parts of 

 the Adirondack region, cutting all of the rocks so far described. 

 They are of all widths up to over 100 feet, though those wider 

 than 30 or 40 feet are exceptional, and few reach those dimen- 

 sions. The larger number have an approximate east and west 

 trend and are nearly vertical. The more common dikes are of 

 a black, flinty, basic rock, diabase; the others are more variable, 

 but are usually quite aoid, red, porphyritic rocks of syenitic 

 make-up. 



They are much more abundant in the northeast than in any 

 other part of the region, being exceedingly numerous in Clinton 

 county and northern Essex, so much so that, if massed together, 

 it would be at once evident that they constitute a very respect- 

 ably large portion of the whole rock mass. Somewhat less 

 than half of Clinton county has Precambric surface rocks, yet 

 some 130 of these dikes are known in the county and there 

 are doubtless many more. Eand hill, Dannemora mountain and 



'Op. cit. P.11J-12J. 



2U. S. Geol. Siir. 16th An. Rep't, i^t. 1, p. 778. 



