HEMITHRENES AND OTHER CALCITIC ROCKS. 55 



either the limestone appears in contact with it, or a "wash-out 

 discloses its former existence, there the rock is serpentine ; where 

 it appears as unaltered rock there is no lime. 



"I find, moreover, that wherever the association can be 

 observed, the lime invariably "is beneath the serpentine. So it is 

 with the loch of Cliff lime and the serpentine of Unst, both of 

 the lime and serpentine beds at Polmally, both of the lime and 

 serpentine beds at Portsoy, at Limehillock, Tombreck, the Green 

 Hill of Strathdon, and Beauty Hill ; and in enumerating these I 

 have named all the most important masses in the country"*. 



We have only one objection to Professor Heddle's view. It 

 would seem that he considers the calcareous matter, resulting 

 from the changes he advocates, to be transferred to beneath the 

 rock from which it was abstracted. But a difficulty strikes us 

 as to what was in the place of the new " calcareous belt" before 

 it was formed. This point seems to have been overlooked. We 

 are quite ready to admit the important bearings of the fact 

 stated by Heddle, that the lime " invariably is beneath the ser- 

 pentine ; " we cannot but suspect, however, that the lime has 

 taken the place of a mass of transmuted serpentine or malacolite. 



The notable shotted structure which frequently characterizes 

 hemithrenes, and in many instances ophites, has an important 

 bearing on the present question, inasmuch as this structure is 

 due to the presence of crystalloids or portions of different 

 minerals possessing a peculiar exterior. 



As far as we have been able to ascertain, Macculloch appears 

 to have been the first to bring the matter before the notice of 

 geologists f. According to his description of the Tiree marbles 

 (one mass of which is "an irregular rock a nodule of 

 limestone, improperly called a bed, lying among the gneiss 

 without stratification or continuity"), they consist of amor- 

 phous or finely granular calcite in pink, white, and other 

 colours, in which are imbedded crystalloids of sahlite, augite, 

 and hornblende, termed in a general way coccolite, separately 

 dispersed, or aggregated in the latter state attaining the size of 

 an orange or larger. Other minerals are present, as mica, ser- 

 pentine, steatite, sphene, &c. The crystalloids are often either 



* Op. cit. pp. 540, 641. Bischof mentions the occurrence of beds com- 

 posed of limestone beneath others formed of mineral silicates. See ' Physical 

 and Chemical Geology,' vol. iii. pp. 304, 30o, 307, 308. 



t Western Islands of Scotland, vol. i. pp. 48-56 (1819). 



