On the Genesis of Some Scottish Minerals. 211 



the '' comby " structure of mineral veins. Hence, also, raises 

 the association of various minerals which have been deposited 

 under different conditions of temperature. 



Lastly, reverting to the statement that mineral veins 

 represent the waning efforts of volcanic action, it must be 

 borne in mind that fault-lissures and volcanic eruptions 

 have been formed at various and widely-separated periods, 

 in Scotland as elsewhere. Hence it might be expected to 

 be the case that the contents of the Scottish mineral veins 

 are of very diverse ages. Some of them may possibly date 

 from as far back as the Devonian Period — though this is not 

 very likely to be the case. Others may have been formed 

 at the close of the Carboniferous Period of volcanic activity. 

 A third set may have originated with the advent of the 

 Keuper Marl. A fourth may date from Miocene times 

 (which appears to be that to which most of the lead- 

 veins of Yorkshire belong). Furthermore, a set of mineral 

 veins formed say, during ISTew Eed times, and subsequently 

 altered by surface agencies, are likely to have afforded 

 channels of egress for the hot springs of Miocene times. 

 Complicated interactions may thus frequently have resulted. 

 Such veins as those of Leadhills and Wanlockhead, as well 

 as the corresponding veins of Galloway and those of the 

 Caldbeck Fells in Cumberland, may well have had a history 

 of the kind just referred to. In that case there is little 

 need for wonder at the varied and complex nature of their 

 mineral contents. 



B 3. Minerals arising from Solfataric Action. 



There is no clear evidence of any Scottish minerals having 

 arisen directly through this agency, unless they are to be 

 found in certain minerals of secondary origin, which occur 

 in the hearts of the great volcanic centres of Tertiary age, 

 such as those of Eum, Mull, Skye, etc. 



B4. Nor is it quite certain that any Scottish minerals, 

 except the Chalcedony in some bands of old Geyserite, can 

 properly be referred to this category. 



