132 G. H. Morton — Range of Carboniferous Species. 



per square incli for every 12" Fahrenheit. Though there are no data 

 that I kaow of on which to estimate the stress set up in gneiss or 

 terra-cotta by a given rise of temperature, we can see that it cannot 

 be otherwise than severe. 



This is the suggestion that I have to make : — The direct heafc 

 of the sun in Brazil must greatly exceed that in this country, and 

 the range between the temperature of the gneissic rocks with the 

 sun in the zenith and the temperature of the same during midnight 

 radiation must be very considerable. Now, according to my studies, 

 this is the very thing wanted to produce disruption. The surface of 

 the gneissic rock becomes greatly heated during the day, but this 

 daily range of temperature cannot extend far into the rock 

 (unfortunately I know of no experiments to determine the depth) ; 

 consequently the difference of expansion between the upper and 

 under layers creates shearing stresses, which, being renewed daily 

 year after year, eventually shears the rock, quarrying out a bed 

 or sheet of a thickness determined by several factors, in the same 

 way that the cement bed of a terra-cotta coping by recurrent daily 

 expansion of the coping is sheared along its whole length. Once 

 the minutest fracture occurs, water and chemical agents, as so well 

 set forth by Professor Branner, detach the large sheet or scale from 

 its parent rock. 



Shortly stated this appears to me to be the origin of these huge 

 exfoliated scales and the " sheets of rock, sometimes 15 feet thick," 

 " utilized by the quarry-men in breaking out blocks of convenient 

 sizes," but which " sheets are more commonly from 2 to 10 feet 

 thick," and " often as thin as a knife-blade." 



It is the recurrent stresses that eventually break up the rock, 

 for on each expansion the rock is strained, and it does not recover 

 its exact original condition on contraction. As field-geologists, we 

 all know that an obdurate rock may resist one great blow of 

 a hammer, bat yields to repeated lesser blows; and it is this daily 

 and yearly recurrent straining of the gneissic rocks of Brazil that 

 in my opinion finally shears oif large plates, and produces the 

 topographic features so characteristic of that country. 



VTII. — The Eange of Species in the Carboniferous Limestone 

 OF North Wales.^ 



By G. H. Morton, F.G.S. 



ATTENTION having recently been directed to this subject, 

 I have been induced to present the results of many years' 

 collecting in the Carboniferous Limestone of North Wales. The 

 formation there presents four well-defined subdivisions, each of 

 them, with the exception of the highest, having distinct lithological 

 characters, viz, : Lower Brown Limestone, Middle White Lime- 

 stone, Upper Grey Limestone, and the Upper Black Limestone. 

 Lists of the fossils have been made, collected more or less con- 

 tinuously along the country from each subdivision. 



1 Eead before tlie British Association, Section C, Liverpool, 1896. 



