180 Canadian Record of Science.. 



The distribution of thsse limestones is represented on the 

 accompanying map. Since they are much softer than the 

 accompanying gneiss, they nearly always occur in depres- 

 sions and are consequently often so concealed by glacial 

 deposits or dense forest that it is hard tc trace them out. 

 The limestones, however, continue just as persistently as 

 the other members of the stratified series. Single beds 

 may be traced for many miles, while certain horizons in 

 the gneiss at which the limestone bands occur, sometimes 

 quite pure, and again rendered more or less impure by the 

 presence of various disseminated minerals or thin layers of 

 gneiss, can be traced as far as the limits of the map. It 

 must here be remarked that many irregularities in form 

 presented by these limestones, must be attributed to the 

 fact that the limestones (as every observer may perceive) 

 under the great pressure to which these rocks have been 

 subjected are much more plastic than the associated rocks. 

 Thin layers of gneiss interstratified with them are often by 

 the folding of the rocks torn asunder into extraordinarily 

 bent and twisted ribbonlike pieces which lie isolated 

 in the limestone so that there results a pseudo con- 

 glomerate. The fact that these limestones are now 

 and then squeezed into cracks in the associated gneisses, 

 led Emmons in his description of the geology of the 

 State of New York, to express the opinion that they were 

 of eruptive origin. The greater plasticity of the limestone 

 as compared with other rocks has also been established, as 

 is well known, by many direct experiments. Since, there- 

 fore, they alternate with the gneiss and follow its strike, 

 and because they are more easily distinguishable than any 

 other of the countless varieties of gneiss, Logan recognized 

 that a careful study of their distribution would furnish a 

 clew for the unravelling of the structure of this or any 

 other Laurentian area in which they occur, and moreover 

 that by the determination of their relations to the anor- 

 thosite rocks, very important data might be obtained 

 concerning the stratigraphical position of the latto/. In 

 investigating that portion of the area which lies to the 



