Short Answer 
Silver mining is significantly more difficult than gold mining due to the need for advanced machinery, substantial investment, and complex extraction processes. In the 19th century, as gold became harder to mine, prospectors faced increasing challenges in accessing deep silver veins, requiring further investment and advanced techniques.
Understand the Difficulty of Silver Mining
The key term that completes the sentence is ‘difficult’. This reflects the unique challenges associated with silver mining which are significantly greater than those for gold. Silver extraction often requires sophisticated machinery and considerable investment, making it a more complex process.
Recognize the Machinery and Investment Requirements
Mining silver typically demands advanced technology and larger financial resources compared to gold mining. The process involved requires miners to invest in:
- Complex machinery for deeper excavation
- Significant financial capital to support operations
- Extended labor due to the intricacies of tunneling
Appreciate the Mining Conditions of the 19th Century
During the mid to late 19th century, the landscape of mining shifted significantly. Individual prospectors could initially mine gold using simple tools. However, as gold deposits were depleted, miners needed to:
- Utilize more advanced machinery for deeper mining
- Expand their operations significantly
- Overcome the physical and financial barriers of accessing silver veins deep inside mountains